Sensor bearings including absolute angle sensors are widely used to serve motion control, electric motor control and commutation, in particular at high temperatures and/or speeds. Such motor application fields could be mining, cement, minerals, drilling of oil/gas, processes in chemistry and mineral refining, off-road electrically driven vehicles, hybrid and electrical cars and trucks, and motors for auxiliary equipment around internal combustion engines such as sensors for the motors/generators of electrically supported turbocompression, electrical air inlet compressor, double clutch actuator motors, turbo variable geometry actuators, turbo waste valve actuators, air heating, cooling and ventilation compressors, water pumps, start-stop motor-alternators, mild and full hybrid electric motor-generators.
Absolute angle sensing devices for use in bearings are usually made of a magnetic ring with at least one north and one south pole, and a collection of two of more isotropically distributed analogue Hall sensors with regular spacing. Adding the signals with the correct weight makes it possible to directly present output voltages representing the sine and cosine of the angle of the (usually) rotating magnet ring with respect to the (usually fixed) sensor collection.
An angle sensing bearings with semiconductor sensors are normally limited to ambient temperatures below the range of 150-175° C. due to the need of highly integrated magnetic sensors as well as analogue and/or digital signal treatment parts produced in silicon (e.g. op-amps, AD converters etc). There are signal processing components available for ambient temperatures beyond 175° C. but they are 100 to 1000 times more expensive than their automotive grade equivalents as used for industrial sensor bearings.
High temperature magnetic sensors are feasible however, usually in the form of Hall plates with sensor areas (in the shape of maltese cross design) in high temperature semiconductors of the GaAs, GaN or SiC instead of Si materials. These Hall elements are today commercially available in GaAs but without any amplification, buffering or on-board signal treatment. The integration level of all analogue and digital processing in Silicon has entailed the massive success and subsequent multi billion pieces/year volumes and thereby low cost silicon Hall sensors and is not available in high temperature semiconductors today.
The cost restrictions applicable in the automotive industry impose strong limits on the possible choices of absolute angle sensors for bearings. For applications where high temperatures are likely to occur such as wheel bearings close to brakes or bearings in internal combustion engines, these limitations do not allow for the use of signal processing elements such as amplifiers and only elementary Hall sensors and passive components can be used.
One of the problems when using high temperature magnetic sensors is that these sensors, e.g. GaAs based Hall elements, have large offset, gain and other variations with respect to the average parameters.
However, the common manufacturing process has the capability to very accurately reproduce these errors in all such Hall elements spread over the same wafer. This means that the Hall bridge resistances, the Hall voltage offset and the Hall gain of these Hall elements may deviate strongly from the typical specification from batch to batch but within one batch of sensors, these numbers will be very constant.